A motor drive device drives a motor that has a plurality of winding pairs. If a failure occurs in a portion of one winding pair or in one “system” that is formed as a combination of one winding pair and an inverter or the like corresponding thereto, the motor continues to be driven by stopping the system that has the failure (i.e., a failed system) and by operating a system that is normal or, in other words, has no failure (i.e., a normal system). Such a motor drive device is disclosed in JP-A-2011-142744 (a patent document 1). Patent document 1 also discloses performing a failure detection, which detects a failure of the motor drive device, before starting an operation of the motor drive device.
When the failure detection is performed for each of the plurality of systems, as described in the patent document 1, a failure detection end time may be different from system to system. In such a case, if a power supply time for starting a power supply is different from system to system (i.e., from one winding pair to the other), during a period in which the power supply is provided for only a portion of the plurality of systems, a rotation torque generated by the portion of the plurality of systems may be different from a predetermined value because of the different failure detection end times of the systems. Therefore, to avoid a torque shortage period in which the rotation torque is different from the predetermined value, patent document 1 discloses the power supply for all winding pairs is started by waiting for a completion of failure detection for all of the plurality of systems.
Further, in patent document 1, a short failure (i.e., a short circuit) of a power relay is detected from system to system in order to turn ON the power relay when no short failure is detected in each of the power relays. Therefore, when a short failure detection time is different from system to system, a power relay turning ON time is also different from system to system.
Further, there may be a case in which a capacitor is disposed in association with an inverter. In such a case, if the power relay turning ON time is different from system to system, the capacitor of the system that has a power relay turning ON time set before other systems has an electric charge stored therein. Such an electric charge in one capacitor may be discharged as a large electric current through a low impedance current path to other systems that have a subsequent power relay turning ON time, when the power relay turning ON time arrives for the other systems with the subsequent time. Such a large electric current may damage a power relay, an inverter, and/or a substrate of the other systems in which the power relay is turned ON later.